1. Technical Field
The present invention relates generally to digital wireless communications systems in which delays of multi-path components of a time-varying fading channel are to be estimated, such as, for example, systems using RAKE receivers in Code Division Multiple Access (CDMA) systems. The invention is particularly suitable for, but is not limited to, receivers that operate in fading environments and have limited processing resources, such as those in Wideband CDMA (WCDMA) terminals.
2. History of Related Art
In wireless communications, a physical channel between a transmitter and a receiver is formed by a radio link. In most cases, a transmit antenna is not narrowly focused towards the receiver and, in addition to a possible direct path, many other propagation paths exist between the transmitter and the receiver. The propagation paths are often caused by reflections from surrounding objects. Rays with similar propagation distances combine at the receiver depending on an instantaneous phase relationship and form a distinct multi-path component. The effect of the combining depends on an instantaneous relationship of a carrier wavelength and distance differences and, in the case of destructive interference, the combining leads to a significant reduction in path-gain magnitude (i.e., fading).
Performance of a CDMA receiver is oftentimes improved if signal energy carried by many multi-path components is utilized via a RAKE receiver. In the RAKE receiver, a number of multi-path components are each assigned a despreader having a reference copy of a spreading code that is delayed equally to a path delay of a corresponding multi-path signal component. Outputs of the despreaders (i.e., RAKE fingers) are coherently combined to produce a symbol estimate.
The RAKE receiver requires knowledge of the multi-path delays and channel-impulse-response values for as many signal paths as possible. To achieve an optimal signal-to-noise ratio (SNR) at an output of a RAKE combiner, signal energy from as many physical paths as possible should be collected. In addition, tracking of as many different physical paths as possible (i.e., higher utilized diversity) tends to significantly increase reception robustness by reducing the probability of a simultaneous deep fade of all tracked signal paths. Simultaneous deep fading of all tracked signal paths is a phenomenon that leads to undesirable block error rate (BLER) degradation.
A propagation channel structure (i.e., absolute and relative delays of individual multi-path components) does not remain constant over time. Due to relative movement of the transmitter, the receiver, and nearby objects, existing path delays change, old paths disappear, and new paths appear. In addition, a frequency offset between the transmitter and receiver often gives rise to a slow clock drift, which may manifest itself as a gradual delay-profile time-axis movement. In order to ensure proper operation of the RAKE receiver, varying delays of all known multi-path components should be tracked, and new paths should be discovered quickly after they appear.
FIG. 1 is a block diagram of a typical RAKE receiver. A RAKE receiver 100 includes a delay estimator block 102, a channel estimator block 104, and a RAKE despreader/combiner block 106. Received data are fed to the delay estimator block 102. The delay estimator block 102 evaluates an impulse response of a channel over a range of possible delays of the channel. A resulting delay profile, which may be a complex delay profile or a power delay profile, may then be subjected to peak detection and detected peak locations reported to the RAKE despreader/combiner block 106 as delay estimates for the multi-path components. The delay estimates are also used by the channel estimator block 104 to estimate corresponding complex channel coefficients by despreading a pilot sequence and possibly filtering results over time to reduce the effects of noise and interference. Channel parameters are estimated in collaboration between the delay estimator block 102, which determines temporal alignment of a despreader portion of the RAKE despreader/combiner block 106, and the channel estimator block 104, which estimates the complex coefficients to be used by a combiner portion of the RAKE despreader/combiner block 106.